SOLAR-DRIVEN LIKE MARS OR ENDOGENIC LIKE ENCELADUS?

46th Lunar and Planetary Science Conference (2015)
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TRITON’S PLUMES – SOLAR-DRIVEN LIKE MARS OR ENDOGENIC LIKE ENCELADUS? C. J. Hansen1 and R. Kirk2, 1Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, Tucson, AZ 85719, [email protected], 2United States Geological Survey, 2255 N. Gemini Dr., Flagstaff, AZ 86001, [email protected]
Introduction: Triton’s young surface with relatively few craters stands out among moons in the solar
system and puts it in a class with Io, Europa, Titan and
Enceladus – other moons with active surface processes
today. Particulate plumes rising 8 km above the surface were imaged by Voyager in 1989, in Triton’s
southern spring [1]. Dark fans deposited on the surface were attributed to deposits from similar, nolonger-active plumes, as shown in Figure 1. The
plumes were subsequently modeled as solar-driven
expulsions of nitrogen carrying particles entrained
from the surface [2, 3].
Triton’s warm interior: Triton’s surface age of
<10 MY is derived from the lack of craters on its surface [4], likely erased by surface yielding, deformation
and viscous relaxation. New models of Triton’s interior suggest that heating is ongoing and could not be a
remnant from Triton’s capture into orbit around Neptune [5]. A liquid mantle was first suggested as a result of Triton’s capture into orbit around Neptune
[summarized in 6]. Later work showed that with a
sufficient ammonia component a liquid layer could
persist to present time [7]. The new model of the interior of Triton shows that the combination of radiogenic
heating with tidal heating due to Triton’s obliquity
could sustain a long-lived subsurface ocean, and sluggish convection, even without invoking substantial
ammonia [5].
Figure 1. Dark fans of material are deposited
across the south polar region of Triton in this Voyager
image. Both of the active plumes can be seen rising
vertically from the surface, then being bent by ambient
winds.
Source of the plumes: Are Triton’s plumes solardriven or do they come from a subsurface ocean? Are
they more like Enceladus or the seasonal gas jets of
Mars?
Solar-driven activity – the Mars analogy. Triton’s
nitrogen atmosphere is in vapor pressure equilibrium
with surface ices, and will form polar caps in the winter. The solar-driven model for Triton’s plumes relies
on a solid state greenhouse forming in/below a seasonal layer of nitrogen ice. A 4 K rise in temperature
causes a 10x increase in vapor pressure, and this temperature difference is easily achieved [3]. The detection of plumes by Voyager in late southern spring is
consistent with the timing expected for solar-driven
jets.
The Voyager imaging team immediately noted the
similarity to fans seen seasonally in Mars’ southern
polar region. The discovery of the fans and modeling
of the plumes on Triton later inspired the solar-driven
model for the origin of the fan-shaped deposits imaged
on Mars’ seasonal CO2 polar caps [8]. This model
postulates that gas from basal sublimation of a seasonal ice layer is trapped beneath impermeable translucent ice. Eventually when the pressure is high enough
the ice will rupture and the gas will escape, entraining
surface particles. The particulates fall out onto the top
of the ice layer in fan-shaped deposits oriented by the
ambient wind. The Mars Reconnaissance Orbiter High
Resolution Imaging Science Experiment (HiRISE)
images, shown in Figure 2, taken every spring have
largely substantiated this model [9].
The combination of HiRISE images and updated
models of the jets have allowed us to quantify parameters such as gas exit speeds (~20-300 m/sec), mass flux
(30-150 gm/sec), height achieved (50-100m), volatile
storage requirements, and lifetimes < 2 hr [10].
Figure 2. Fans deposited on the seasonal CO2 polar caps every martian spring are captured in this
HiRISE image (ESP_011960_0925).
46th Lunar and Planetary Science Conference (2015)
Eruption from the interior – the Enceladus analogy. We now have another possible comparison, with
the Cassini discovery that Saturn’s moon Enceladus
spews water vapor and ice particles from fissures
across its south pole [11, 12, 13], shown in Figure 3.
Enceladus showed us that it is possible to have regionally confined geophysical activity, likely driven by
tidal energy [14, summarized in 15].
The most recent Cassini radio science data show
that there is a subsurface gravity anomaly consistent
with a body of liquid water 30 to 40 km below the
south pole, extending up to ~50S latitude [16]. Other
recent observations give a source size of 9 m [17], with
vapor exiting at speeds up to 1-2 km/sec in collimated
jets [18], consistent with the postulate that warm vapor
from a subsurface ocean exits through a nozzle-like
openings to the surface [19]. Vapor mass flux is on the
order of 200 kg/sec [18]. Solid particle flux is ~50
kg/sec [20].
Figure 3. Cassini images show ice particles being
erupted from fissures across Enceladus’ south pole
[13].
Summary: The solar-driven model has been the
accepted explanation for many years for Triton’s
plumes. The distribution of fans is consistent with that
model, the timing of the eruptions coincided with
southern spring, and it is eminently plausible in terms
of energetics. Challenges with gas storage and the
required layered surface structure were considered
surmountable [3].
More recent data and models however motivate a
re-examination of the source of Triton’s plumes. The
age estimate for Triton’s surface and recent tidal models incorporating obliquity were not available in the
Voyager era. Study of Mars’ jets has allowed us to
characterize and quantify solar-driven processes on
that planet. The discovery of tidally-driven eruptions
confined geographically on Enceladus and measurements such as vapor mass flux and exit speeds have
expanded possible scenarios for Triton. The vapor
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mass flux in particular, estimated at up to 400 kg/sec,
is more similar to Enceladus than to the jets at Mars.
Triton’s plumes reach 8 km altitude, erupting
through an ambient atmosphere, before being carried
away horizontally by the ambient wind. Although this
can be achieved with solar-driven plumes, perhaps
Triton’s eruptions come from a deeper source. An
interesting test will be provided by New Horizons
Pluto observations. Pluto does not experience obliquity tides and is thus unlikely to have a young surface
similar to Triton [5]. It does however have a nitrogen
atmosphere in vapor pressure equilibrium with surface
ice, that will form polar caps in the winter. If we see
fans and/or plumes at Pluto in the north polar region
now experiencing spring it will bolster the solar-driven
hypothesis.
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